In the oil and gas industry it is conventional to directly or indirectly mount a flow head such as a rotating flow head on the top of a wellhead or a blowout preventer (BOP) stack. The rotating flow head, more commonly known as a rotating control device, serves multiple purposes including sealing off tubulars of a tubing string, moving in and out of a wellbore and accommodating rotation thereof. Tubulars can include a kelly, pipe or other drill string components. The rotating flow head is an apparatus used for well operations and diverts fluids from the wellbore, such as drilling mud, surface injected air or gas and produced wellbore fluids, including hydrocarbons, into a recirculating or pressure recovery mud system.
Operations performed on a well that is not under pressure or flowing need not seal around tubing string as there is no risk of wellbore fluids exiting the wellbore under pressure. In such conditions, flexible conduit, such as a cable or wireline, is simply inserted downhole to provide an electrical connection between downhole logging tools and a surface unit. For wells that are under pressure, sealing around both the tubing string and cable is required. However, conventional sealing elements cannot seal around a tubular and a cable at the same time. Thus, necessitating the stoppage of flow of wellbore fluids and relief of wellbore pressures before further operations such as wireline operations can begin.
Often, underbalanced well operations require an additional flexible tubing or conduit, such as a wireline or cable, to be run downhole alongside a tubing string and connected to a downhole measurement tools. This requires sealing around the tubing string as well as the cable.
As standard rotating flow heads are not designed to seal around a tubing string and a cable running alongside the tubing string, wells under pressure, such as underbalanced wells, are therefore usually killed before operations commence. Killing wells introduces risk of damaging the well and/or reducing the capabilities for gathering data of the wells by logging tools.
Operations requiring the controlled entry of a flexible tubing string (ie. logging tools pushed down into a well on a drill string due to high angles of the well or wells under pressure), in order to avoid having to kill the well and risk damage thereto, require sealing around the tubular as well as sealing around the cable run alongside and adjacent a tubing string. Such operations enable downhole tools to be conveyed on the tubing string while also maintaining an electrical connection to a surface unit using a standard wireline cable.
One example of such an operation is the use of electrical submersible pumps (ESP) at a downhole end of a drill string. The ESP is run in the wellbore with a power cable running between the pump and the rig floor through the rotary table, adjacent or alongside the tubing string.
Another example can be operations involving the conveyance of downhole tools in a well using drill pipe tubulars until just above the bottom of the well. A cable side entry sub is then incorporated into the drill string, the cable side entry sub adapted to allow a cable to access the interior annular space of the drill string. The cable is rigged up at surface to the side entry sub for entering the inside or bore of the drill string. The cable is then run down inside the drill sting and further connects, via a wet connect, to the tools already downhole. The cable is tied up or fixed at the side entry sub and both the cable and drill string are simultaneously conveyed down to perform logging operations. The positioning of the side entry sub is such that it always stays inside the casing while the downhole tool may be within an uncased open hole.
A standard feature of a tough logging condition system (TLC) is that a certain length of cable, equal to the length of the logging interval at a minimum, ends up being outside that portion of the drill pipe located between the drill rig floor or wellhead and down to point in the drill string where the cable enters the drill pipe, i.e. the side entry sub.
In vertical wells, once underbalanced drilling is completed, the well can be logged using conventional logging techniques utilizing surface pressure control systems rigged up through the standard rig blow out prevention stack at the wellhead to accurately determine the reservoir productivity. Supply of N2, if required, can be provided by a parasitic string inserted for this specific purpose.
However, in horizontal and high-angled wells, conventional TLC technique, as used in over-balanced drilling environment suffers, from a limitation as a certain cable section, equal in length to the interval being logged, must be kept outside of the drill pipe. The cable section is located between rig floor and the downhole cable side entry sub which cannot be sealed around as standard rotating flow heads are not designed to seal around a pipe with a wire outside it. Any attempt to do so, using conventional rotating flow heads, could damage the cable and jeopardize the whole operation. This means that advanced service logging operations such as high resolution imaging, production logging measurements, such as downhole flow rates, phase hold ups and zonal contributions from reservoir and others are not available using LWD or memory option, cannot be performed with a standard surface set up, which is a serious disadvantage for the exploration and production operator.
In some cases coil tubing with electric cable could be an option however the ability of coil tubing to push a heavy suite of open hole logging tools all the way to total depth in a long horizontal or high angled open hole is a shortcoming, as well as the added complexity, risk and investment needed to carry out such an operation.
There is a need for a system and a method to introduce a cable into a wellbore alongside a drill string and to seal the drill string and the cable during wellbore operations involving wells under pressure.
There is a need for a system and method to log a high-angled underbalanced well without killing the well.
There is a need for a system and method for sealing around a tubing string run downhole in a wellbore and cable run adjacent the tubing string in the wellbore.